Allow me to run with this briefly.
Incest is lmost universally considered immoral. Now there are very practical reasons why it's not a good idea. My position is that those, by virtue of a random throw of the genetic dice, thought 'Hey, why not..?' have been removed from the gene pool. And those, by dint of a different roll of those dice thought it not such a good idea, have their genes propogate through society. They were selected for, to use an evolutionary term. So now we all consider it to be somehow wrong, even if we weren't aware of the biological problems.
Now if biology had worked out differently and breeding within a family was evolutionary advantageous, then those who thought it was a good idea would be selected for and those who didn't would be removed from the pool. And then we would all think it was wrong to have sex with someone outside your family group.
There is so much that we puzzle about why we think is so right or so wrong that are simply evolved characteristics of homosexual sapien. God given? Hey, go for it. I prefer 'nature did it'.